Have you ever driven past a yard where their yard has a long trench dug from their house to the street? Many times this is for the plumber to replace either the sewer line or the water line to the house. These are the main lines and out of the house. The water line is the clean water that comes into the house from the street and feeds all your faucets and water heater.

The sewer line is what it sounds like, it takes all the sewer water that goes down your drain and takes it to the street where it connects to the mainline where it goes to the treatment facility.